1. Field of the Invention
This invention relates to piezoelectric transducers and particularly to those operating by shear mode deformation of piezoelectric element.
2. Description of Related Art
Piezoelectric transducers have a wide application. They can work as sensors, in which they generally convert a mechanical energy (displacement, acceleration, force, pressure, stress and stress) into an electrical signal (direct effect). Piezoelectric transducers can also be used as actuators or resonators, in which they convert an electrical energy into mechanical (reverse effect).
Piezoelectric transducers are well known for their exceptional bandwidth and dynamic range. Symmetry, rigidity, amplification, lightweight, and shear mode are often identified as design attributes which deliver high performance. Simple construction with few parts and capability of mass production are often identified as desirable characteristics for low cost devices. Notwithstanding that much effort has been put forth to achieve these attributes and characteristics in piezoelectric transducers, efforts have been thwarted because many of the attributes are mutually exclusive. For example, stress amplification often results in a reduction in rigidity and the resultant frequency bandwidth. Additionally, simple stress amplifying designs known as bimorphs and bender beams have precluded the use of shear mode piezoelectric elements which provide high performance with critical temperature insensitivity. It has remained a constant goal of the industry to produce a simple design that delivers high quality and a robust transducer performance.
Broad utilization of piezoelectric-based transducers in the past has been hampered by the associated cost of manufacture. Traditional designs are both labor and component intensive. In the case of piezoelectric accelerometers, for example, piezoelectric beam design accelerometers have disproportionately low strength and a low frequency band. Miniature surface-mounted piezoelectric accelerometers exhibit low resolution.
Automation has brought about the introduction of the silicon micromachined (MEMS) accelerometer. The manufacturing of these devices is automated and they can be produced at low cost, but MEMS accelerometers employ beam type elements having limited resolution, low dynamic response and low frequency range.
Piezoelectric accelerometers of an annular shear type design are an example of a piezoelectric sensor recognized as being suitable for high volume, low cost production because of their simplicity, symmetry and because they require relatively few parts. However, known annular shear type designs suffer from drawbacks which prevent high performance and even higher volume production necessary for achieving a lower production cost, and more widespread use. There are two principal drawbacks associated with these devices. The first is the substantial machining required and the resulting high cost of component manufacturing in order to achieve an interference fit. The second is the use of ordinary solders or adhesives which have been proposed in order to address the first drawback.
Some accelerometer designs, such as those disclosed in U.S. Pat. Nos. 4,075,525 issued to Birchall, and 4,941,243 issued to Cleveland, require expensive conical parts with costly surface finishes. An annular shear accelerometer of the type disclosed in published patent application WO 91/06012, despite its simple design, does not recognize or address the challenges associated with developing a commercially viable low cost assembly of the element using solder. Technology for piezoelectric shear accelerometers has evolved to using more than one crystal in order to achieve interference fit designs. The necessity for more than one crystal increases the price of the sensor.
In general, the joining of annular parts of annular shear accelerometers is subject to several difficulties. One known method for joining together an accelerometer base, ceramic tube, and seismic mass includes joining the parts together by a tight fit and maintaining all other parts together by friction. Such a method requires the use of very close dimensional tolerances which hampers or precludes high volume production. One particular problem associated with such structure is the difficulty of securing mechanical contact of all curved surfaces at all annular locations of the interfitting parts. Conical or tapered parts require costly surface finishes to provide high linearity (&lt;1 %) and resonant frequency (&gt;20 kHz).
Known prior piezoelectric sensor constructions sometimes employ a conductive epoxy compound to fill in gaps and adhere parts together. In the case of the annular shear accelerometer it is common to fill in annular gaps between interfitting portions of the annular seismic mass, the ceramic piezoelectric tube, and the annular, housing with epoxy. Conductive epoxy compounds, however, exhibit low strength, stiffness and further, after hardening, they often contain many voids. It is hence difficult in such devices to control the quality of bonding and the resulting linearity and bandwidth of the sensor. Further, there are Theological problems such as material deformation and flow during and after manufacturing which cause unacceptable changes in sensor performance.
To overcome problems associated with epoxy bonding, many attempts have been made at solder bonding, without a good understanding of the potential sensor performance problems that may and do result from the use of solder bonding. Problems are often encountered due to solder shrinking after its solidification. Standard Sn--Pb solder alloy, after solidification, exhibits about 3-4% shrinkage. As a result, solder gaps may form upon solder solidification, which introduce a random distribution of high intensity residual mechanical stress. Under residual stress, very important properties of piezoelectric ceramics, such as the piezoelectric constant and relative dielectric constant, can change. This is very apparent in symmetrical shear sensor designs since the random residual stresses act in both axial and transverse directions. For example, a lead zirconate titanate piezoelectric ceramic tube may be used in an annular shear accelerometer construction. Before soldering, the crystal has a capacitance of 300 pF. After joining of the accelerometer parts using a solder alloy (63%Sn-37%Pb), the crystal may have a capacitance value anywhere from 120 pf to 300 pf, and the value obtained in the final product is not susceptible of prediction such that the change in value can be accounted for in the design of the accelerometer.
To solve these problems, attempts have been made to select piezoelectric materials that exhibit much greater independence of characteristics from stress, such as barium titanate, quartz and gallium orthophosphate. However, these materials have the distinct disadvantage of having very low piezoelectric charge constants compared to the traditional material formulations of lead zirconate titanate.
The use of solder joining technology has often led to low electrical insulation resistance of piezoelectric sensing elements at temperatures above 90.degree. C., which introduces another problem. To this end, for optimal operation, the piezo-ceramic element of a sensor should have an insulation resistance more than 10.sup.10 ohms. Insulation resistance between electrodes less than 10.sup.8 ohms can result in an increase in bias voltage and/or noise of signal conditioning electronics and, consequently, the sensor cannot operate as desired. The source of low resistance discovered in connection with the present invention is the flux used in soldering. During soldering, a small amount of flux vapor may become trapped within a sensor. In the temperature range of about 100 to 125.degree. C., this flux vapor decreases the electrode insulation resistance between electrodes of the ceramics from greater than 10.sup.10 ohms to less than 10.sup.8 ohms. This problem may arise with any type of flux, even a so-called "no clean" flux.
Finally, it is often cited in the prior art that solder joining is abandoned, avoided, or improved upon because of the limitation on operating temperature that the solder joining imposes on the operating range of a piezoelectric sensor. While it is technically true that many piezoelectric materials have potential operating temperature ranges exceeding common soldering compounds, it is also true that the vast majority of piezoelectric sensors incorporate signal conditioning circuitry in the sensor. Signal conditioning circuitry generally sets the upper temperature limit of the sensor to 121 degrees Celsius.